Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Conventional ion sources consist of a chamber, usually formed from graphite, having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture through which the plasma is extracted to form the ion beam. The gas is ionized by a source of excitation such as a resistive filament or a radio frequency (RF) antenna located within or proximate the chamber. The plasma density, and hence the output current of the extracted ion beam, may be increased by increasing the power applied to the source of excitation.
Increasing the input power applied to the excitation source, however, affects beam characteristics other than beam current. For example, input power is one factor which determines the relative amounts of various atomic and molecular species that constitute the plasma. Accordingly, this characteristic is closely coupled to the beam current and the two cannot be varied independently. Thus, with known ion sources, varying the beam current, which is necessary to determine the precise amount of dosage for a particular implant process, is not possible without altering the plasma constituency.
Some ion implantation systems include mass analysis mechanisms such as beam line magnets that remove undesirable atomic and molecular species from the beam which is subsequently transported to the workpiece. In such systems, the mass analysis mechanism can compensate for variances introduced into the beam constituency as a result of changes made to the beam current. Thus, altering the beam current does not present a significant problem.
In ion implantation systems where no mass analysis occurs, however, the problem of variable beam constituency remains. For example, in applications for implanting large surface areas, such as flat panel displays, a ribbon beam ion source is often utilized. An example of such an ion source is shown in U.S. Ser. No. 08/756,970 and U.S. Pat. No. 4,447,732. A plurality of exit apertures provides the capability for adjusting the width of the ribbon beam. Each of the plurality of exit apertures outputs a portion of the total ion beam output by the ion source. Beam portions output by apertures located between surrounding apertures overlap the beam portions output by those surrounding apertures. However, in such a ribbon beam system, no mass analysis of the ion beam is performed.
Accordingly, it is an object of the present invention to provide an ion source in which the output beam current may be altered independently of the beam constituency.
It is a further object of the present invention to provide such an ion source for use in ion implantation systems that do not include mass analysis mechanisms.
It is still a further object of the present invention to provide a mechanism for an ion source which provides a wide operating range of output begin currents, while maintaining the constituency of the plasma generated within the source.